Deforestation and Coastal Erosion: A Case from East Brazil

J. Addad, M. A. Martins-Neto

Abstract

Many beaches from the eastern Brazilian coast are in erosive conditions. A coastal reach around the town of Alcobaca, located at the estuary of the homonymous river, was monitored through daily observations of meteorologic, oceanographic and sedimentologic parameters during 10 months. Two points are of special interest in the Alcobaca dynamics: 1) the erosion phenomenon is restricted to the last 30 years, when 30 to 40 meters of the beach have been lost; and 2) beaches located on both sides of the estuary are subjected to erosion. During most of the observation time, littoral drift was southward as a consequence of the NE wave field. Erosion occurred when a NE-wind intensification or a wind change to SE, associated with the arrival of a cold front, coincided with the high tide, generating a beach scarp. During spring tides, the beach scarp reaches the permanent vegetation zone. The destruction of these previously stabilized areas induces an irreversible migration of the shoreline. The coastal erosion at Alcobaca is considered to be related to a disequilibrium in the Alcobaca River outlet. Beach erosion on both outlet sides indicates that both drift directions are sediment-starved and that the retention capacity of the Alcobaca River outlet has been amplified, supplanting the ability of both drift directions to bypass the sediment across it. The fact that the ebb-tidal delta in the river outlet has grown over the last few decades, concurrent with the erosive phenomenon, also corroborates this hypothesis. Patterns of heavy minerals content and the granulometric distribution of the sands around the estuarine spit also support the enhanced retention model, providing evidence for sand trapping at the ebb-tidal delta of the estuary outlet. The increase of the hydraulic retention effect, termed here hydraulic mole, which causes the beach erosion, is here interpreted as due to changes in the Alcobaca River characteristics, mainly the increase of its suspended-load concentration, as a result of intense deforestation of the Atlantic rain forest. Therefore, we consider that the ultimate responsibility for the erosion at the Alcobaca area lies with the deforestation and its direct consequences, and not the wind-waves-littoral drift dynamics, which act merely as reequilibration agents.